Various techniques have been devised for actuating or firing a perforating gun in a subterranean well. A bar or "go-devil" may be dropped through the interior of the tubing string for mechanically firing the gun, or for breaking a ceramic disc to significantly alter tubing fluid pressure to the gun, and thereby hydraulically firing the gun. A signal may be sent downhole through an electric line to initiate the firing of the gun, or to release stored electrical energy for firing the gun. In many applications, however, neither a go-devil nor an electric line is a preferred technique for firing the gun, partially due to problems associated with reliability of the gun firing, post-gun firing difficulties associated with using tubing fluid pressure or an electric line to fire the gun, and the expense of running the electric line.
Firing of a perforating gun by altering hydraulic pressure in the annulus between the tubing and the casing has long been an acceptable alternative. Annulus fluids above the packer are customarily isolated from the production fluids by the packer, and annulus pressure can be easily controlled at the surface using readily available pumps. Accordingly, many operators prefer techniques for altering annulus pressure to reliably and inexpensively fire a downhole perforating gun. In many such applications, however, it is also preferred that the packer above the perforating gun be set prior to firing the gun, so that formation fluids and pressure are initially isolated from the annulus between the tubing and the casing above the set packer from the moment the gun is fired.
In order to achieve hydraulic firing of a gun by regulating annulus pressure above the set packer, annular flow passageways positioned radially inward of the sleeve-like packer body and isolated from the tubing bore have been utilized. These flow passageways transmit fluid pressure from the annulus above the packer to the hydraulic firing head of the gun, thereby permitting the reliable and inexpensive firing of the gun. Such flow passages, however, create significant drawbacks which have heretofore limited the commercial acceptance of these packers, and accordingly of the related technique for hydraulically firing perforating guns below a set packer.
One of the drawbacks of prior art packers used for hydraulically firing guns by controlling annulus pressure relates to reduced flow capability through the tubing string. Each section of conduit between the formation and the surface of the well preferably has an internal diameter which is as large as practical up to the nominal diameter of the tubing, so that there are no significant restrictions or "chokes" along the tubing string. The outer diameter of the unset packer sealing elements must also be closely controlled so that the packer can be easily passed through a casing only slightly greater than the diameter of the tubing. Also, the inner diameter of the packer body preferably is not significantly less than the bore diameter of the tubing above or below the packer, as explained above. Accordingly the permissible radial "thickness" of the packer between the interior surface of the packer body and the external surface of the packer sealing elements when in the unset condition must be closely controlled. This "maximum flow capacity" feature of the packer and accordingly of the tubing string does not generally exist, however, when an inner mandrel is provided within the packer body for forming an annular passageway to transmit fluid pressure from the annulus between the tubing and the casing above the set packer, between the inner mandrel and the packer body, and thence to the hydraulic firing head of a gun.
Another drawback relating to prior art packers used for hydraulically firing a perforating gun is that various equipment cannot be easily and reliably passed through the packer bore if one or more flow lines are provided interior of the packer body. It may be desirable to pass various downhole tools, such as a wireline perforating gun, through the tubing string and the bore of the set or unset packer within the well bore. Such a gun can, however, easily get "hung up" on small tubing positioned radially interior of the packer body, thereby causing an expensive retrieval or "fishing" operation, or adversely affecting the reliability of the downhole equipment.
The disadvantages of the prior art have been overcome by the present invention, however, and improved methods and apparatus are hereinafter disclosed for reliably firing a perforating gun positioned below a set packer in a subterranean well by transmitting a pressure signal from the annulus between the tubing and the casing above the set packer to the perforating gun below the set packer.